1. Field of the Invention
The present invention relates generally to fluid level sensing systems and which the level control is accomplished through the use of a float whose motion is transmitted to a pneumatic or electric controller which is connected to a valve and, specifically, to an improved liquid level controller having a pilot valve for sending an output signal pressure to a pneumatically operated process valve for controlling the valve.
2. Description of the Prior Art
The oil and gas, chemical and other industries utilize process valves for the control of process fluids which are operated by means of a pneumatic or electrical control signal. The pneumatic control for such valves typically includes a pilot valve whose function is to send an output signal pressure to the pneumatic controller, which either opens or closes the process valve. In the typical prior art system, the level of liquid in a tank or other container is sensed with a sensing element or float that is in communication with the liquid in the container and which transmits a force or movement to a control device that is situated outside the container. The force or displacement so sensed is a measure of the change in liquid level.
For instance, in the case of an oil and gas separator tank, a liquid level controller is provided which uses a float or displacement type sensor to transmit changes in the liquid level in the tank to a pilot valve outside the vessel. The pilot valve signals a process control or discharge valve in the discharge outlet from the vessel to open or close the discharge valve in response to the liquid level within the vessel.
Supply gas pressure was generally taken from the production gas and routed to the pilot valve. When the liquid level in the vessel is within the desired limit, the supply gas is vented through the pilot valve to the atmosphere. When the liquid level rises sufficiently to change the position of the float, the flapper applies a force to the pilot valve so that the supply gas is diverted within the pilot valve to thereby provide a control signal to the discharge valve which allows liquid to flow from the vessel.
Many of the prior art liquid level controllers require right or left hand mounting which requires that both mountings be available in inventory. Also, their conversion between such mountings requires extensive reworking of the mounting and the components. Liquid level controllers are described as "direct" and "throttling" in operation if an increase in level in the tank results in a proportional increase in outlet pressure from the controller. Controllers were described as "indirect and throttling" in operation if an increase in level within the tank resulted in a proportional decrease in outlet pressure from the controller. Controllers which operated in the "snap-on or snap-off" mode featured a sudden increase in output pressure with level increase in direct mode and a sudden decrease in output pressure with level increase for indirect operating mode.
The prior art devices were difficult to reconfigure for either direct or indirect signal operation and for snap-on and snap-off or throttling mode and lacked a convenient mechanism for varying the sensitivity of the devices. It was often difficult to reach the internal components of such devices for repair and reconfiguration. Conversion of the prior art devices from "snap-on and snap-off" to "throttling" mode of operation generally required disassembly and substitution of alternative parts. Where a supply gas filter was installed, the filter was difficult to access and clean.
An object of the present invention is to provide an improved liquid level controller which can be easily and quickly converted between direct and indirect action and which has a full range of sensitivity adjustment in both settings.
Another object of the invention is to provide a liquid level controller having an improved pneumatic pilot which is simply and easily switchable between a snap-on and snap-off and a throttling type action.
Another object of the invention is to provide an improved liquid level controller which utilizes an easily adjustable spring counterbalance to accept displacement sensors of various configurations.
Another object of the invention is to provide such a liquid level controller having a supply gas filter housing with an improved filter arrangement which facilitates maintenance or replacement of the filter element.
Another object of the present invention is to provide an improved liquid level controller which is simple in design, economical to manufacture and which is dependable in operation.